Quick erection bridges are typically used for such purposes as forestry operations where temporary bridges are required. Forestry roads are frequently built only to provide temporary access during active logging operations. Once the logging operations are completed, the roads are often abandoned and bridges are removed. Thus it is highly desirable to provide bridges which may be quickly and economically erected and which can be dismantled and re-used at another point after logging operations are completed. Military operations as well require bridges which can be quickly erected with a minimum of equipment. Such bridges are also used for emergency purposes when permanent structures are washed out or collapse.
One problem associated with such quick erection bridges is the method of making the initial span across the creek, ravine or other gap to be crossed. It is possible to use cranes to position relatively heavy beams across the gap. However, this means that high load capacity cranes must be available and frequently this is not feasible or economical at remote sites. A common method used to make the initial span is to place a relatively lightweight "nose" at the leading edge of the bridge which is then moved across the distance to be spanned. The nose rests on a roller on the opposite side of the span and supports the permanent bridge structure as it is moved across the gap. The nose does not contribute to the permanent structure and is simply removed after the permanent structure spans the gap. The transport of the nose to the site and the assembly of the nose and disassembly of it are all tasks which increase the expense and time involved. U.S. Pat. No. 4,520,523 to Fitzgerald-Smith discloses a bridge module which is initially moved into place by means of a crane. Nose girders are connected to the front end of the bridge. The entire unit is then rolled over a roller beam, the nose girders being removed as they reach the far bank.
British Pat. No. 1,266,820 shows the use of rollers to reduce bending in the bottom chords of trusses while being launched.
Earlier U.S. patents of general interest include U.S. Pat. Nos. 3,491,391 to Soffge, 4,493,122 to Echtler, 4,120,065 to Sivachenko, 4,521,932 to Parramore, 3,707,011 to Launay, and 2,878,498 to Gollnow.